The delivery of high-velocity particle streams for surface preparation, such as the removal of coatings, rust and millscale from ship hulls, storage tanks, pipelines, etc., has traditionally been accomplished by entraining particles in a high-velocity gas stream (such as air) and projecting them through an acceleration nozzle onto the target to be abraded. Typically, such systems are compressed-air driven, and comprise: an air compressor, a reservoir for storing abrasives particles, a metering device to control the particle-mass flow, a hose to convey the air-particle stream, and a stream delivery converging-straight or converging-diverging nozzle.
The delivery of high-velocity particle streams for the cutting of materials, such as the "cold cutting" (as opposed to torch, plasma, and laser cutting, which are "hot-cutting," thermal-based methods) of alloys, ceramic, glass, and laminates, etc., has traditionally been accomplished by entraining particles in a high-velocity stream of liquid (such as water) and projecting them through a focusing nozzle onto the target to be cut. Typically, such systems are high-pressure water driven, and comprise: a high-pressure water pump, a reservoir for storing abrasives particles, a metering device to control the particle mass flow, a hose to convey the particles, a hose to convey high-pressure water, and a converging nozzle within which a high-velocity fluid jet is formed to entrain and accelerate the particle stream onto the target to be cut.
Whether the particle stream is delivered for the purpose of surface preparation or cutting, the mechanism of action, known to the skilled artisan as "micromachining," is essentially the same. Other effects occur, but are strictly second order effects. The principle mechanics of micromachining are simple. An abrasive particle, having a momentum (I), which is the product of its mass (m) times its velocity (v), impinges upon a target surface. Upon impact, the resulting momentum change versus time (m.times.dv/dt) delivers a force (F). Such force applied to the small-impact footprint of a sharp particle gives rise to localized pressures, stresses and shear, well in excess of critical material properties, hence resulting in localized material failure and removal, i.e., the micromachining effect.
As evidenced by the above discussion, since the specific gravities of commercially significant abrasive particles are within a narrow range, any major increase in their abrading or cutting performance must come from an increase in velocity. Second, not only is velocity important, but, for surface preparation applications, the particles must contact the surface in a uniformly diffuse pattern, i.e., a highly focused stream would only treat a pinpoint area, hence requiring numerous man-hours and large quantities of abrasive to treat a given surface. Third, ideally, the particles should impinge upon the surface to be treated and not upon each other. Yet for cutting applications, a focused stream is desirable in order to erode deeper and deeper into the target material and, in some applications, to sever it.
The skilled artisan in the particle stream surface preparation and abrasive cutting art desiring to perfect an apparatus or method for surface preparation or cutting, faces a number of challenges. First, the amount of abrasive particles required per area of coating removed can be very high, which in turn means not only higher costs of use, but higher clean-up and disposal costs.
Second, the use of abrasive particles in the conventional dry blasting process described herein generates tremendous amounts of dust, both from the particles themselves and from the pulverized target material upon which the particles impinge. Such dust is highly undesirable because it is both a health hazard and an environmental hazard. It is also a safety and operations-limiting concern to nearby machinery and equipment. To ameliorate this, some systems add water at a low pressure to wet the particles immediately before ejection from the apparatus' nozzle assembly. Yet the water has the undesirable side effect of reducing the velocity of the abrasive particles, which, in turn, reduces the effectiveness of the particles for their intended purpose (i.e., coating removal or materials cutting). Adding water has the additional undesirable side effect of causing the abrasive particles to aggregate and form slugs which also severely diminishes their effectiveness. It is the shared belief in the industry that water cannot be added to a dry air/particle stream without diminishing the particle velocity. This belief has been corroborated by extensive testing. Yet the addition of water to the air/particle stream is essential for many applications to suppress dust generation, and, may in fact be the only remedy that complies with applicable environmental, health and occupational/operational safety regulations.
Third, currently available particle stream abrasive cutting systems (using abrasive particles to cut low-cost materials such as steel, concrete, wood, etc.) require a much higher power input relative to other current methods such as: torch, plasma, laser, or diamond-blade cutting, for instance. Hence the inferiority of abrasive cutting relative to other methods is not due to cutting efficacy, but rather cost. Air or water jet-driven abrasive cutting requires a higher power input, making it cost- prohibitive for most applications other than for special situations which mandate cold-cutting and/or contour cutting of thermally sensitive materials.
Therefore, the problem facing the skilled artisan is to design an apparatus or method that delivers an evenly distributed, diffuse stream of abrasive particles to a surface to be cleaned (or a focused stream of abrasive particles to a surface to be cut) at the highest velocity, at the lowest possible power input, and without the generation of unacceptable levels of airborne dust.
The most straightforward solution, which is increasing the velocity of the particles, is problematic. This is done conventionally by entrainment of the particles in air, though air is an ineffective medium to accelerate particles over a short distance, due to its low relative density and practical-length limitations for an operator-deployable entrainment/acceleration nozzle. That is, the particles, beyond a certain velocity, do not continue to accelerate with the air, but move more slowly than the air, in a slip stream. Particle velocity, when driven by an air stream, is further reduced because often, water must be introduced into the air/particle stream to "wet" the particles to reduce airborne dust. This water, upon entrainment within the particle/air stream, results in a further reduction of the stream's velocity--often a substantial reduction.
Therefore, a crucial need in the art would be met by the development of a method or apparatus that delivers an evenly distributed, diffuse stream of abrasive particles to a surface (to be cleaned) or a focused stream to a surface (to be cut) at the highest possible particle velocity, at the lowest possible power input, and which does not generate unacceptable levels of airborne dust.